In Long Jingtian's understanding, the true "Marrow Cleansing" could be achieved through three methods.
One was to take special medicinal herbs that were over a thousand years old, using their strong medicinal power to expel impurities from the body.
Another was for martial artists to utilize their own True Essence Force in conjunction with a special cultivation technique to reach the purpose of "Marrow Cleansing."
The third method involved other experts using an Ambrosia Baptism-like technique to help martial artists force the impurities out of their bodies.
However, all three of these methods were difficult to achieve.
Medicinal herbs with a thousand years of medicinal age were extremely rare in the world.
Cultivation techniques that allowed a martial artist to perform Marrow Cleansing did not seem to exist in the Secular Ancient Martial World.
Experts who knew how to perform Marrow Cleansing on other martial artists with the art of Ambrosia Baptism were also hard to find.